Heat-sensitive recording element and composition



United States Patent 3,460,964 HEAT-SENSITIVE RECORDING ELEMENT AND COMPOSITION Kenneth Royal Dunham, Rochester, N.Y., assignor to Eastman Kodak Company, Rochester, N.Y., a corporation of New Jersey No Drawing. Filed Nov. 19, 1964, Ser. No. 412,332 Int. Cl. B41m 5/18, 5/26 US. Cl. 117-369 18 Claims ABSTRACT OF THE DISCLOSURE A heat-sensitive composition which can be used to prepare heat-sensitive recording elements comprises an aqueous solvent, a metal nitrate, an organic phenolic, furan or lignin compound, and optionally a thickener, a stabilizer, a sensitizer or similar agents.

This invention relates to an improved heat-sensitive or thermographic copy system based on inorganic salts, and more particularly based on certain metallic nitrates in combination with certain organic compounds that enter into color-forming reactions under the influence of heat.

In conventional thermographic front-printing procedures, the graphic original to be copied such as a typewritten letter or document, printed form or page, line drawing, etc., is placed in pressure contact with the heatsensitive surface of a copy-sheet, and the assembly then subjected to a heat source, for example, to infrared radiation. The copy-sheet thereby becomes colored only in the image areas, thus reproducing the original. For this purpose, inorganic salts in combination with certain organic compounds have been proposed. For example, one of the proposed methods employs as the heat-sensitive layer a mixture comprising phenolphthalein or aniline hydrochloride with potassium nitrate or nickel nitrate. However, this method has a number of serious shortcomings, which have limited its use to just certain very specific applications. For one thing, the coating solution gives a dark brown precipitate within seconds of mixing the components. Then, on standing, it grows steadily darker and after about 2030 minutes, becomes inactive and does not give a heat-sensitive coating. Furthermore, the papers coated promptly after preparation of the solution, are generally brown in color and tend to have an offensive odor. Moreover, contamination of the original document is quite severe.

I have now found that mixtures comprising a liquid medium or solvent, certain metal nitrates and certain organic compounds are heat-sensitive, becoming deeply colored on exposure to a heat source such as infrared radiation. I have further found that when such mixtures or compositions in unexposed condition are applied to suitable supports, for example, a fibrous sheet material such as paper, and dried, the elements so formed are likewise heat-sensitive and eminently suitable as recording or copy sheets for reproduction of original typewritten letters, documents, printed sheets, line drawings, etc. Furthermore, such heat-sensitive recording elements or sheets are nearly white in appearance, stable under ordinary keeping conditions and cause little or no document contamination even after many passes through a T hermofax machine (trade name for a commercial thenmographic machine). The images reproduced on exposure of an original document in contact with the heat-sensitive recording elements or sheets of my invention are characterized by being, ordinarily, dark brown in color on an essentially white background, sharp in definition and relatively stable.

It is, accordingly, an object of the invention to pro- 3,460,964 Patented Aug. 12, 1969 vide a new and improved heat-sensitive composition which can be readily applied to a support material. Another object is to provide an improved heat-sensitive recording element or sheet having at least one layer thereon of the above composition. Another object is to provide an improved heat-sensitive recording element or sheet of the above type which reproduces exact copies of original typewritten or printed letters, documents, etc. in dark colors against a nearly white background by a bireflex exposure to a heat source such as infrared radiation. Another object is to provide a process for preparing these new heat-sensitive compositions and heat-sensitive recording elements or sheets. Other object will become apparent from the description and examples.

In accordance with the invention I prepare my new and improved heat-sensitive compositions and heat-sensitive recording elements by intimately mixing together at least one metal nitrate and at least one reactive organic compound, i.e., capable of entering a color-forming reaction with the nitrate compound under the influence of heat, such as a phenolic compound, a furan compound, a lignin compound, and the like, in a suitable liquid medium which is a solvent for the above mixture of compounds, preferably an aqueous system such as a mixture of Water and a water-soluble organic solvent including lower alkanols, ketones, etc. (e.g., methanol, ethanol, acetone, 1,4-dioxane, etc.). The proportions of water and solvent are not critical, but preferably the mixtures comprise about from 40-90% by weight of water and from 60-10% by weight of the organic solvent. The compositions prepared as above are then applied or coated onto fibrous sheet material which is preferably a high grade paper of low lignin content by conventional methods such as spraying, brushing, dipping, using a coating knife, etc. The resulting coating is ordinarily allowed to air dry. The heat-sensitive recording element thus produced can be used directly for preparing reproductions of original documents, etc. or stored for several weeks or more before using without appreciable loss in activity.

However, not all nitrate compounds give satisfactory or practical heat-sensitive compositions and heat-sensitive recording elements or sheets. For instance, the nitrates of sodium, potassium, barium, strontium, cerium, lead and lanthanum (e.g., NaNO KNO Ba(NO C6(NO3)3, Pb(NO3)2, SI'(NO3)2, LH(NO3)3, etc.) definitely are not operable in my invention. Ammonium nitrate also is not operable. Salts other than nitrate likewise do not give satisfactory results. Thus metal sulfates are not active. Metal chlorides such as ZnCl produce images; however, these images are unstable and the background becomes dark quite rapidly. In contrast, operable high quality heat-sensitive compositions and heatsensitive recording elements or sheets are obtained in accordance with the invention with a select group of compounds such as the nitrates of silver, copper, zinc, magnesium, mercury, aluminum, yttrium, thorium, bismuth, chromium, iron, cobalt, and nickel (e.g., AgNO 3)2, a)2 3)2, 3 z, 3)3, 3)a, 3)4, 3)3, 3)2, 0 93, Co(NO and Ni(CO Silver nitrite (AgNO is also operable and produces satisfactory compositions and thermographic recording elements or sheets. Those of the above compounds that also contain water of crystallization in their formulas are also included.

In regard to the reactive organic compound component, here too not all phenolic, furan, lignin, etc. compounds give satisfactory or practical heat-sensitive compositions and heat-sensitive recording elements or sheets with the above-mentioned select group of metal nitrate compounds. For example, phenol and catechol are not satisfactory components because they cause poor contrast and inferior keeping qualities. Most furan compounds also are inoperative. As to the lignin, the types of papers containing sufficient lignin to give even relatively poor images, without need for lignin substitutes are generally lower grade, unsized papers. High grade papers are preferably used in my invention and for its reason only certain lignin substitutes in conjunction with the above-mentioned select group of nitrate compounds, are suitable for producing satisfactory heat-sensitive compositions and heat-sensitive recording elements or sheets. Suitable reactive organic compounds for practicing the invention are thus strictly limited to a select group including syringaldehyde, lignin sulfonic acid, vanillyl alcohol, vanillin, vanillil, guaiacol, resorcinol, gallic acid, Z-naphthol, 2,4- dihydroxybenzoic acid, 3,5-dihydroxybenzoic acid, 2- naphthyloxy acetic acid, 2-furanacrolein, 2-furanacry 1ic acid, bis(p-nitrophenyl) disulfide and furoin.

The concentration of the metal nitrate compounds in the heat-sensitive compositions of the invention can vary quite widely, but preferably from 5 to 30% while that of the reactive organic compound can vary about from 2 to 25%, based in each case on the total weight of the solution. The usual practice is to dissolve the metal nitrate salt in the aqueous medium and then add thereto the reactive organic compound dissolved in, for example, acetone or methanol. One hundred ml. of the solution sufiices to coat on the average of from 50 to 80, 9" x 12 sheets having satisfactory heat-sensitivity and good image recording. This latter result depends somewhat on the paper stock used. For example, on thin sheets of paper stock normally employed for bireflex exposures in thermographic systems, image intensity appears undesirably weak because of light transmitted through the paper. This disadvantage can be overcome, if desired, by employing paper stock having a layer of a brightening agent on at least one surface such as aluminum oxide, titanium dioxide, baryta, lead oxide, etc. Advantageously, an organic acid which is normally solid at room temperature can be added to increase the speed of the heat-sensitive compositions and heat-sensitive recording elements. For example, a typical heat-sensitive composition containing 20% by weight of Al(NO will produce coated papers giving relatively weak images at a setting of 2 on a Thermofax machine. However, on addition to the composition of 5 to 15% of citric acid, tartaric acid, mandelic acid or benzenephosphonic acid, the sensitivity of the coated paper is sufficiently increased to give dark strong images at a machine setting of 6. Acids such as 2,4-dinitrophenylacetic, m-nitrobenzoic, picolinic and 1,23,4- butanetetracarboxylic acids also produce speed increases but of somewhat lower order than that of the aforementioned acids. If desired, a thickening agent can also be incorporated into the composition to increase the viscosity and improve the coating ease. Suitable thickeners include hydrophilic colloids such as gelatin, albumin, agar-agar, gum arabic, alginic acid, etc., or a hydrophilic synthetic resin such as polyvinyl alcohol, polyvinyl pyrrolidone, a cellulose ether, etc. The concentration of the thickener can range from about 0.5 to 5% depending on'the particular thickening agent and the desired result. The heat-sensitive compositions can be also contain other materials that alter or improve the qualities of the heat-sensitive recording elements or sheets produced therewith for example, infrared absorbents to increase sensitivity, humectants or hydrated salts to control the moisture content, stabilizers to improve image permanence and to prevent increase of background color, or activity controlling compounds such as 2,2-(phenylimino)diethanol, aminodietha- 1101, etc. which function to maintain the heat-sensitive recording elements or sheet in a uniformly active condition for several weeks or more, and thereafter for some time still operable at a gradually reduced sensitivity level. The activity controlling compound is preferably present in an amount of about from 1-5 based on the total weight of the composition or solution.

4, The following examples illustrate more specifically the manner whereby I practice my invention.

Example I.Preparation of heat-sensitized sheet A thin sheet of acid resistant, heat-conducting paper was coated on one side by sprayingwith a brightening composition comprising titanium dioxide, and dried. This coatedsurface was then overcoated by spraying thereover a thin coating ofthe following heat-sensitive composition:

Aluminum 'nitrate 'g 10.0 a-Resorcyclic acid g 7.0 2,2-(phenylimino)diethanol g 3.0 Acetone ml 28.0 Water to bring total to ml The coating was then allowed to air dry.

In place of the titanium dioxide, there can be substituted in the above example any other of the mentioned brightening agents such as aluminum oxide, baryta, lead oxide, etc. Also, the application of the heat-sensitive coat can also be done by brushing, dipping, etc. or by using a coating knife, when the compositions contain a thickening agent such as the mentioned hydrophilic materials.

Example -II.Pr0duction of image by heat A printed page to be copied was covered with the sensitized sheet prepared as described in Example I, with the unsensitized side of the copy paper in contact with the original. The assembly was then passed through a Thermofax Secretary machine (trade name of a commercial thermographic machine) at a speed in the region of the buff setting. A legible dark brown image of good stability was obtained on a white background. This was repeated with other samples of heat-sensitive sheets prepared as in Example I, excellent images being obtained on freshly coated sheets as well as on sheets aged for one month before exposure.

By substituting either the aluminum nitrate or the mresorcylic acid, or both, in above Examples I and II with any other of the mentioned nitrates or reactive organic compounds, other species of heat-sensitive compositions and corresponding heat-sensitive recording elements or sheets can be readily prepared. Such sensitized sheets likewise have good keeping properties and produce legible and stable dark images on white backgrounds. Thus, in the procedures of Examples I and II, combinations of nickel nitrate and a-resorcylic acid; the combination of coppernitrate and guaiacol; the combination of aluminum nitrate and gallic acid; etc., likewise give satisfactory heat-sensitive compositions or solutions and corresponding excellent heat-sensitive recording sheets.

The chemistry involved in image formation in my invention is not definitely known. The evidence gathered to date indicates, however, that the color-forming reaction apparently involves oxidation of the reactive organic compounds with the metal nitrates on exposure to a heat source, possibly followed by chelation with the metal ion. Only the metal nitrates previously mentioned as suitable in the practice of the invention give this desirable color-forming reaction.

The invention has been described in detail with particular reference to preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention as described hereinabove and as defined in the appended claims.

What I claim is:

1. A heat-sensitive composition comprising (1) a compound selected from the group consisting of copper nitrate, zinc nitrate, magnesium nitrate, mercury nitrate, aluminum nitrate, yttrium nitrate, thorium nitrate, bismuth nitrate, chromium nitrate, iron nitrate, cobalt nitrate and nickel nitrate, (2) an organic compound selected from the group consisting of syringaldehyde, lignin sulfonic acid, vanillyl alcohol, .vanillin, vanillil, guaiacol, resorcinol, gallic acid, Z-naphthol, 2,4-dihydroxybenzoic acid, 3,5-dihydroxybenzoic acid, Z-naphthyloxy acetic acid, Z-furanacrolein, Z-furanacrylic acid, bis(p-nitrophenyl) disulfide and furoin, (3) an aqueous solvent system comprising a mixture of water and a water-soluble organic solvent, and (4) an activity controlling compound selected from the group consisting of 2,2'-(phenylimino) diethanol and aminodiethanol.

2. A heat-sensitive composition of claim 1 composed of from 5 to 30% by weight of said (1), from 2 to 25% by weight of said (2), from 1 to 5% by weight of said (4), and the remainder to make a total of 100% of said (3).

3. A heat-sensitive composition of claim 1 wherein a hydrophilic colloid material is included.

4. A heat-sensitive composition of claim 1 wherein a normally solid, water-soluble carboxylic acid is included.

5. A heat-sensitive composition of claim 2 wherein said (1) is aluminum nitrate, said (2) is a-resorcylic acid, said (4) is 2,2'-(phenylimino)diethanol, and said (3) is a mixture consisting of from 40-90% by Weight of water and from 60-10% by Weight of acetone.

6. A heat-sensitive composition of claim 2 wherein said 1) is nickel nitrate, asid (2) is a-resorcyclic acid, said (4) is 2,2'-(phenylimino)diethanol, and said (3) is a mixture consisting of from 40-90% by weight of water and from 60-10% by weight of acetone.

7. A heat-sensitive composition of claim 2 wherein said (1) is zinc nitrate, said (2) is resorcinol, said (4) is 2,2'-(phenylimino)diethanol, and said (3) is a mixture consisting of from 40-90% by weight of water and 60- by weight of acetone.

8. A heat-sensitive composition of claim 2 wherein said (1) is copper nitrate, said (2) is guaiacol, said (4) is 2,2'-(phenylimino)diethanol, and said (3) is a mixture consisting of from 40-90% by weight of water and from 60-10% by weight of acetone.

9. A heat-sensitive composition of claim 2 wherein said (1) is aluminum nitrate, said (2) is gallic acid, said (4) is 2,2'-(phenylimino)diethanol, and said (3) is a mixture consisting of from 40-90% by weight of water and from 60-10% by weight of acetone.

10. A heat-sensitive recording element comprising a paper support coated with a heat-sensitive composition comprising (1) a compound selected from the group consisting of silver nitrate, copper nitrate, zinc nitrate, magnesium nitrate, thorium nitrate, bismuth nitrate, chromium nitrate, iron nitrate, cobalt nitrate and nickel nitrate, (2) an organic compound selected from the group consisting of syringaldehyde, lignin sulfonic acid, vanillyl alcohol, vanillin, vanillil, guaiacol, resorcinol, gallic acid,

Z-naphthol, 2,4-dihydroxybenzoic acid, 3,5-dihydroxybenzoic acid, Z-naphthyloxyacetic acid, 2-furanacrolein, 2- furanacrylic acid, bis(p-nitrophenyl)disulfide and furoin, (3) an aqueous solvent system comprising a mixture of water and a water-soluble organic solvent, and (4) an activity controlling compound selected from the group consisting of 2,2-(phenylimino)diethanol and aminodiethanol.

11. A heat-sensitive recording element comprising a paper support coated on one surface with at least one layer of heat-sensitive composition of claim 2.

12. A heat-sensitive recording element comprising a paper support coated on one surface with at least one layer of the heat-sensitive composition of claim 3.

13. A heat-sensitive recording element comprising a paper support coated on one surface with at least one layer of the heat-sensitive composition of claim 4.

14. A heat-sensitive recording element comprising a thin sheet of acid resistant, heat-conducting paper support coated on at least one surface with a brightening agent and having at least one surface thereof coated .with a layer of the heat-sensitive composition of claim 5.

15. A heat-sensitive recording element comprising a thin sheet of acid resistant, heat-conducting paper support coated on at least one surface with a brightening agent and having at least one surface thereof coated with a layer of the heat-sensitive composition of claim 6.

16. A heat-sensitive recording element comprising a thin sheet of acid resistant, heat-conducting paper support coated on at least one surface with a brightening agent and having at least one surface thereof coated with a layer of the heat-sensitive composition of claim 7.

17. A heat-sensitive recording element comprising a thin sheet of acid resistant, heat-conducting paper support coated on at least one surface with a brightening agent and having at least one surface thereof coated with a layer of the heat-sensitive composition of claim 8.

18. A heat-sensitive recording element comprising a thin sheet of acid resistant, heat-conducting paper support coated on at least one surface with a brightening agent and having at least one surface thereof coated with a layer of the heat-sensitive composition of claim 9.

No references cited.

MURRAY KATZ, Primary Examiner US. Cl. X.R.

1062l; ll7-36, 155, 156; 26041 

